This invention relates to III-V semiconductor devices and in particular to improvements in the current confinement in such devices.
Semiconductor devices known in the prior art comprise several layers of III-V semiconductors. It is often desirable to form areas of high resistivity within certain layers so as to define conducting stripes for current flow. Such is the desire in many semiconductor lasers.
A basic structure of the semiconductor laser is the double heterostructure (DH) which typically includes an n-type GaAs substrate, and a layer of low-refractive-index, high bandgap n-type AlGaAs deposited onto the substrate. Upon this layer, a thin layer of high-refractive-index, low bandgap GaAs or AlGaAs is formed followed by a layer of low-refractive-index, high bandgap p-type AlGaAs. The thin, high-refractive-index GaAs or AlGaAs layer is referred to as the "active" layer. It is this layer in which the stimulated emission occurs.
In order to obtain high efficiency from such a device, the active volume must be kept very small. The higher bandgap layers confine the carrier recombination in the vertical direction by trapping the carriers in the lower bandgap active layer. By constructing the device such that the active layer is very thin, one can minimize the active area in one dimension. However, other techniques must be utilized in order to laterally confine the active volume.
In the design of lasers of the foregoing type, several methods have been used for lateral current confinement. One such method involves the bombardment of the top epitaxial layers with protons in order to form resistive regions therein which laterally confine current to a semiconductive stripe. By narrowing the stripe, one can make the active region narrow. This type of structure is generally known from U.S. Pat. No. 3,824,133 issued to D'Asaro et al, July 16, 1974. However, due to the lack of strict lateral current confinement in all layers, such a structure tends to produce a light beam in which the lateral position of the light output along the active layer varies. This can be an undesirable feature for certain applications of lasers, for example, in trying to guide the beam through an optic fiber.
Also known in the prior art is the idea of fabricating narrow conducting stripes at the substrate -n ternary layer interface. W. T. Tsang and R. A. Logan reported on such a structure in their article, "Lateral-Current Confinement in a GaAs Planar Stripe Geometry and Channeled Substrate Buried DH Laser Using Reverse-Biased p-n Junctions", which appeared in the Journal of Applied Physics, May 1978, Vol. 49, page 2629. This scheme involves the formation of a p-type layer on an n-type substrate, then etching channels through the p-type layer into the n-type substrate. Another n-type layer is grown over the p-type layer and into the channels, thus forming p-type regions between the n-type layer and substrate. This causes the boundaries of the p-type regions to act as blocking junctions thereby defining narrow conductive stripes in the channel regions. However, this current confinement scheme requires two separate growth procedures to form the epitaxial layers, separated by fabrication steps to form the blocking junctions.
Another method for confining current which involves the formation of p-type regions between n-type layers is generally known from U.S. Pat. No. 3,984,262 issued to Robert Burnham and Donald Scifres. This patent involves the formation of resistive regions in a diode laser by diffusion of an impurity into a substrate of a selected conductivity type so as to form secondary p-n junctions defining a conductive stripe, with subsequently formed epitaxial layers providing a primary p-n junction at the boundary of the active laser region. Forward biasing of the primary p-n junction results in reverse biasing of the secondary p-n junctions, and current is confined to the stripe. However, during the subsequent growth of the epitaxial layers for such a device, dopants tend to diffuse in the lateral direction and vertical direction, thereby making it difficult to define a current confining stripe. Further, because epitaxial growth temperatures tend to be equal to or greater than diffusion temperatures, the dopants may diffuse upward faster than the layers can be grown, thereby making it difficult to maintain primary p-n junctions.
Accordingly, it is the primary objective of the present invention to provide a III-V semiconductor device with controlled current confinement.
Another objective of the present invention is to provide a technique which allows for the fabrication of III-V semiconductor devices with controlled current confinement.